Although applicable to a number of fields, the present invention was conceived relative to providing a low cost, in-oil switch for an hydraulic oil filter. It is common knowledge that such filters are provided with a filtering element through which oil is forced under pressure from a pump. Periodic replacement of the filter element is necessary because it becomes plugged or clogged with foreign matter removed from the oil. Failure to attend to such replacement can cause serious damage due to contamination.
Signal devices for alerting an operator to the plugged or clogged condition are known. Such devices commonly provide for movement of a magnet or conductor which is then sensed. For example, U.S. Pat. No. 3,654,414 shows a piston moveable by a higher pressure fluid which in turn moves a finger between a pair of switch arms thereby making or breaking a circuit. The problem is that the switch requires a pressure chamber for operation of the piston and a switch housing wherein the finger member operates a switch. Similar signaling devices provide for movement of a magnet which is then sensed by a separate device.
Another category of indicators operates in conjunction with a by-pass valve. For example, U.S. Pat. No. 3,644,915 shows a terminal connected through a spring to a valve element which is in contact with the valve housing to complete a circuit. Movement of the valve element breaks the conductive path. In addition to the various fluid communication channels, the device requires hardware for connection through a wall to make electrical connection with the spring. U.S. Pat. No. 2,879,892 shows a somewhat similar device which does not need the electrical connection through the valve wall, but does require an additional chamber for operation of the valve element. Although bypass valve signaling mechanisms can have a purpose of providing a warning when a filter becomes plugged or clogged, the mechanism is primarily a valve with electrical attachments, as needed. These devices are often less desirable for signaling purposes than switch devices not having a valve function since valve operation is required for signaling, resulting simultaneously in the bypass of the filter element. It is more desirable to signal before the bypass element opens so the filter element can be changed to avoid the need for bypass.
Thus, known art shows switch devices more complex than desirable, and bypass devices providing valve function which may be inappropriate for many applications.